There are 176 known and verifiable impact craters on Earth. It’s not a huge number, granted, but for photographer Stan Gaz each one represents a cataclysmic event, the moment our world was assailed by an asteroid or comet.

“Looking at these things,” Gaz says, “you realize the vulnerability of everything. [An impact] could change the world we live in so quickly, in less than a second.” That notion was so compelling that he spent six years traveling the globe to document craters for his book Sites of Impact (Princeton Architectural Press, $60).

To capture the photos, Gaz leaned his Hasselblad Superwide out the open doors of helicopters and a small plane over the Arctic, the Southwest, Australia, and Africa. His harnesses, he says, were usually “little more than car seat belts,” and one time, he looked up to find his pilot sound asleep. But he got the shots, stark black-and-white images that stare down seemingly moments after impact when the world is scattered with ash, when it’s hard to tell if we’re seeing the end or just another beginning.